Cyanide free bath for electrodeposition of silver

ABSTRACT

The specification discloses an improvement in the electrodeposition of silver from an aqueous cyanide-free bath which comprises the use organic nitrogen containing compounds having at least two nitrogen atoms and having a molecular weight of at least 300, and a compound of sulfur or selenium in which the sulfur or selenium is in an oxidation state of minus one or minus two. The combination is used as an activator in the bath.

Cyanide-free aqueous baths for electrodepositing silver coatings areprovided. Such baths have thiosulfate solutions of silver.

The invention relates to a cyanide-free aqueous bath for theelectrodeposition of silver coatings.

Alkali cyanide bright silver baths have long been known. But, because oftheir high toxicity, there is a need for less poisonous baths. Thecyanide-free electrolytes proposed until now in this connection have,however, found no application in the practice because of the followingdisadvantages: their instability, their economic inefficiency or theinferior properties of the resultant coatings deposited therefrom. Theelectrolytic deposition of silver from cyanide-free thiosulfatesolutions is also known (See Elektrochemie, Vol. 45, No. 10 (1939), pp.757-759). Such solutions, containing silver chloride and sodiumthiosulfate, have also proved unsatisfactory for the deposition ofbright silver, and could not be improved substantially by the additionof cyanide, phosphate, turkey-red oil or other capillary-activesubstances and colloids. Also, changing the silver or thiosulfatecontent or the pH value had no basically improving effect; but even ledto inferior coatings, to passivation of the anodes, or to accelerateddecomposition.

It is, therefore, the object of the present invention to develop acyanide-free, thiosulfate containing silver bath which avoids thedisadvantages of the known baths and permits the electrodeposition ofbright silver coatings having good mechanical and electrical propertieswithout passivation of the anodes and which are extremely stable.

This is achieved, according to the invention, by a cyanide-free aqueousbath containing as essential constituents a silver compound and athiosulfate and optionally conventional bath constituents, and which ischaracterized in that it contains additionally at least one organicnitrogen compound with at least two N atoms and a molecular weight ofover 300, a sulfur or selenium compound of the oxidation degrees "minusone" or "minus two" respectively, or mixtures thereof.

As bath there is preferably used an aqueous thiosulfate solution whichcontains soluble silver or silver-complex compounds. Such soluble silvercompounds are, for example, silver sulfate, nitrate, chloride, bromide,cyanide, thiocyanate, oxide, carbonate, sulfamate, acetate, and nitrate.The soluble silver complex compounds are, for example, the alkali silvercyanides such as KAg(CN)₂, alkali silver thiocyanate such as K₂ Ag(SCN)₃or K₃ Ag(SCN)₄ ; alkali silver sulfites, such as Na₃ Ag(SO₃)₂, or silvercomplexes with nitrogen-containing compounds, for example, ammonia,amines or polyamines. It has been found to be particularly favorable toadd the silver to the bath in the form of its preformed thiosulfatecomplexes, for example, Na₃ Ag(S₂ O₃)₂, Na₄ Ag₂ (S₂ O₃)₃. Alternatively,the thiosulfate may be added to the bath directly in the form of itsammonium and/or alkali salts, such as the sodium and potassium salts, orof its adducts of thiosulfuric acid or thiosulfuric ion with basiccompounds, for example, the amines or the polyamines.

The production of the mentioned complex compounds may occur in a wellknown manner. Thus, Na₃ Ag(S₂ O₃)₂ can be produced by reacting, forexample, as ammoniacal silver nitrate solution with sodium thiosulfateand precipitating the formed complex with potassium nitrate and alcohol.The use of small proportions of cyanide-containing salts is possiblewithout disadvantage in the bath according to the present inventionbecause due to the relatively high thiosulfate content a conversion soontakes place with the formation of thiocyanates.

The concentration of the silver in the bath may be from 0.5 g to 60 g/l,preferably 20 g to 40 g/l bath liquid. It has proved advantageous if themolar ratio of silver to thiosulfate referred to their ions Ag⁺ and S₂O₃ ² ⁻ is at least 1:3, preferably 1:4 to 1:6. The quantity ofthiosulfate ions, e.g. in the form of Na₂ S₂ O₃.5H₂ O may then be, forexample, 4 to 800 g/l, preferably 180 to 550 g/l bath liquid. The pHvalue of the bath may be between 5 and 14, preferably between 7 and 11,and is adjusted to the desired value in the usual manner.

Suitable, as additions to be used according to the invention, areparticularly the characterized nitrogen-containing organic compoundswith at least two N atoms and a molecular weight of over 300.

Such nitrogen compounds are, for example, polyamines, namelypolyethylene polyamine and other N-containing poly-molecules, which maybe linear as well as branched. These compounds are known or can beproduced by methods known in the art, for example, by polymerization ofpolyethylenimine, polypropylenimine or by polyaminoalkylation of ammoniaor of primary or secondary amines. Especially suitable are nitrogencompounds whose molecular weights are from about 300 to over 50,000,preferably from 500 to 20,000. A very good effect show those solublepolynitrogen compounds used which are formed by the reactions well knownof epihalohydrins (glycerin dichlorohydrin) with ammonia, amines orpolyamines.

As starting products for the production of these compounds there areused those nitrogen compounds which have at least one amino group, suchas ethylene diamine, tetraethylene pentamine, propylene diamine,N,N-dimethyl aminopropylene, N(n-butyl)-propanediamine-1,3, dipropylenetriamine, gamma, gamma-diaminopropylthio ether,N,N-bis-(4-hydroxybutyrul)-dipropylene triamine, tetramethylene-diamine,hexamethylene diamine, N-(1,6-hexanediamine)-3-pyrrolidone, spermine,4,4'-dipiperidyl, aminopyridine, hexamethylenetetramine and polyimines.

Other nitrogen compounds to be used according to the invention areheterocyclic compounds, for example polyvinyl-2-pyridine andpolyvinyl-4-pyridine, as well as quaternary polyammonium compounds,which also are well known or which can be produced by well knownmethods, such as by conversion of the above named compounds withquaternizing agents, namely alkyl halides, alkylene halides, alkylsulfates, esters of arylsulfonates or epihalohydrine.

Another possibility for the production of quaternary polyammoniumcompounds to be used according to the invention is offered by thereaction of alkylene halides with amines or polyamines, such as thereaction of 1,4-dichlorobutane with tetramethylethylene diamine. Also,the nitrogen compounds may be added in the form of betaines orsulfobetaines or in the form of ethoxylated compounds.

It has been found, furthermore, that in addition to thesenitrogen-containing compounds certain sulfur and/or selenium compoundsfurnish excellently bright and ductile silver coatings. Sulfur orselenium compounds very suitable, in particular, are those in which thesulfur or selenium has the oxidation degree of "minus one" or "minustwo."

By oxidation degree must be understood the so-called oxidation number orcharge value, i.e., that charge which an atom would have in a moleculeif the latter were composed of ions only.

Suitable compounds having these oxidation degrees are, for instance,those of the general formulas:

I. Compounds of the general formula

    R.sub.1 -- X.sub.1 -- X.sub.2 -- R.sub.2

where R₁ and R₂ are identical or different and represent hydrogen, aunivalent metal equivalent or an organic radical, R₂ being in addition,the groups --CN or --SO₃ Me, and X₁ and X₂ are identical or differentand represent a sulfur or selenium atom, and Me is a metal atom.

II. Compounds of the general formula

    R.sub.1 ' -- X -- R.sub.2 '

where R₁ ' and R₂ ' are identical or different and represent hydrogen, aunivalent metal equivalent or an organic radical, R₂ ' represents inaddition the groups --CN or SO₃ Me, and X is a sulfur or selenium atomand Me a metal atom.

III. Compounds of the general formula ##STR1## where R₁ " and R₂ " areidentical or different and represent a univalent metal equivalent or anorganic radical, R₁ " is in addition hydrogen, R₃ an organic radical, Xa sulfur or selenium atom, and Z an acid radical.

IV. Compounds of the general formula ##STR2## where R₁ '" and R₂ '" areidentical or different and represent an organic radical, X is a sulfuror selenium atom and Y a non-metal atom.

It should be noted that the metallic disulfides and diselenides, whereR₁ and R₂ represent a metal atom, are predominantly ionogenic, so thatthe formula should more correctly be written, for example, as

    R.sub.1 R.sub.2 X.sub.2

or as

    R.sub.1 .sup.+R.sub.2 .sup.+(X--Y).sup.-.sup.-

but this is known to the specialist.

As univalent metal equivalents enter into consideration herein, forexample, Na, K, or Ca/2, etc.; organic radicals that may be named, arealiphatic, aromatic, cycloaliphatic and araliphatic radicals, which mayoptionally also be substituted and/or interrupted by one or more heteroatoms, such as oxygen, nitrogen or sulfur, and/or one or more heteroatom groups, such as

    >S = O

or ##STR3## as well as the aracyl radical and the CN group.

Substituents for said organic radicals are, for example, halogen atoms,as chlorine, bromine, etc., hydroxyl radicals, alkyl radicals, as methyland ethyl etc., aryloxy radicals, as phenoxy, etc., acyloxy radicals, asacetoxy, etc., the nitro and cyano group, the carboxy and sulfonic acidgroup in free or functionally modified form, e.g., as esters, or assalts, heterocyclic radicals, as tetrahydrofuryl, etc., as well as theradicals

    H.sub.2 N -- CO -- NH -- CO --

or

    Ar -- NH -- CO --

for the onium compounds, the usual acid radicals of the onium compoundsmay be used, such as the inorganic acids, preferably of hydrohalicacids.

Compounds to be used according to the invention are the following:##STR4##

These compounds are known and can be produced by the known methods.Application of the nitrogen, selenium and sulfur compounds to be usedaccording to the invention can take place in concentrations of about 10⁻⁸ to 0.5 mol/lt liquid bath, the compounds being added either alone orin a mixture.

As further additives the bath may contain the usual constituents, whichare, for example, conducting salts such as ammonium sulfate or alkalisalts of inorganic or weak organic acids, which as sulfuric acid,sulfurous acid, carbon dioxide, boric acid, sulfaminic acid, acetic acidor citric acid, etc., as well as pH-regulating substances, preferablythe organic and/or inorganic buffer mixtures suitable for this purpose,such as disodium phosphates, carbonate, borate and acetate.

The anodic and cathodic current efficiencies of this bath are almost100%, resulting in an extremely high stability of the bath compared withother thiosulfate-containing baths. Thus, at a load of 400 Ah/liter noadverse change in the operation of the bath is noted in two months, andthe silver anodes dissolve very evenly without developing passivityphenomena.

Moreover, the coatings deposited from the bath of the invention are farsuperior to the precipitates separated from cyanidic electrolytes. Thusthe coatings exhibit great hardness (with values HV₀.01 :140 to 180kp/mm²) as well as a very good specific electric conductivity of about40 m Ohm per mm², whereas from the cyanidic baths normally used eitheronly precipitates of great hardness but low electric conductivity or oflow hardness but good conductivity can be deposited.

Another advantage of the coatings deposited from the bath of theinvention is their better wear resistance, improved by the factor of 1.5over coatings obtained from known electrolytes.

Baths of the composition according to the invention are suitable forsilverplating in engineering, especially electrical engineering, as wellas for decorative purposes. Thus, for example, electric contacts, plugstrips, metallized plastics and ceramic materials can be silver-platedwith them to great advantage. In this application especially theremarkable electrical properties (conductivity, contact resistance) andthe excellent abrasive strength at improved tarnish stability play animportant role. This is significant, in particular, because more usersare looking for an equivalent substitute for the ever-more expensivegold-plating.

The advantages in decorative silver-plating (jewelry, silverware,instruments) reside mainly in the remarkable luster combined with anextremely warm tone and very good ductility of these silver coatings.

Especially favorable for the economic use of the electrolyte of theinvention is the fact that in addition to the above mentionedproperties, it possesses also a very good dispersion capacity. Thissupports its universal applicability, that is, it can be usedadvantageously for the silver-plating of rack as well as drum ware. Thisholds true for baths with or without movement of the ware, permittingthe use of higher current densities at increasing movement. As a rule,however, the anodic current density should expediently not exceed avalue of 1.5 A/cm².

The following examples will illustrate and form part of the invention.

EXAMPLE 1

    ______________________________________                                        Silver as     Ag.sub.2 SO.sub.4                                                                         40      g/liter                                     Sodium thiosulfate                                                                          Na.sub.2 S.sub.2 O.sub.3.5H.sub.2 O                                                       360     g/liter                                     Sodium pyrosulfite                                                                          Na.sub.2 S.sub.2 O.sub.5                                                                  18.3    g/liter                                     Sodium sulfate                                                                              Na.sub.2 SO.sub.4                                                                         20      g/liter                                     Polyethylenimine                                                                            mol.wt.>1000                                                                              0.08    g/liter                                     pH value 5.7                                                                  Temperature 20-23°C                                                    ______________________________________                                    

Coatings deposited from this bath have high luster. The micro-hardnessaccording to Vickers is HV₀.01 = 174 kp/mm². Analogous results areobtained with polyethylenimine (mol.wt. > 1000) and polypropylenimine(mol.wt. >1000).

EXAMPLE 2

    ______________________________________                                        Silver as     Ag.sub.2 CO.sub.3                                                                         28      g/liter                                     Sodium Thiosulfate                                                                          Na.sub.2 S.sub.2 O.sub.3.5H.sub.2 O                                                       370     g/liter                                     Polyethylenimine                                                                            mol.wt.>1000                                                                              0.15    g/liter                                     pH value 10.2                                                                 Temperature 25°C                                                       ______________________________________                                    

This electrolyte furnishes high-luster coatings. Micro-hardnessaccording to Vickers is HV₀.01 = 160-165 kp/mm². Specific electricresistance is approximately 3.5 μ Ohm cm.

Analogous results were obtained with polyethylenimine (mol. wt. 1000)and reaction products of polyethylenimine with acetic acid anhydride orpropane sultone.

EXAMPLE 3

    ______________________________________                                        Silver as      Na.sub.2 Ag(S.sub.2 O.sub.3)2                                                             30      g/liter                                    Sodium thiosulfate                                                                           Na.sub.2 S.sub.2.sub.-3.5H.sub.2 O                                                        250     g/liter                                    Sodium tetraborate                                                                           Na.sub.2 B.sub.4 O.sub.7.10H.sub.2 O                                                      20      g/liter                                    Sodium sulfite Na.sub.2 SO.sub.3                                                                         20      g/liter                                    Reaction product of                                                           1,4-dichlorobutane with    0.2     g/liter                                    tetramethylethylene                                                           diamine                                                                       pH value 10.0                                                                 Temperature 20-25°C                                                    ______________________________________                                    

The silver coatings deposited from this bath are remarkable forexcellent ductility and high luster combined with extremely warm tone.The micro-hardness according to Vickers is HV₀.01 = 150-160 kp/mm², andthe specific electric resistance 2.5 μOhm cm. Similar results areobtained with reaction products of 1,4-dibromobutane or1,3-dibromopropane and tetraethylethylene-diamine ortetramethylpropylene-diamine.

EXAMPLE 4

    ______________________________________                                        Silver as     Na.sub.3 Ag(S.sub.2 O.sub.3).sub.2                                                           23     g/liter                                   Sodium thiosulfate                                                                          Na.sub.2 S.sub.2 O.sub.3.5H.sub.2 O                                                          105    g/liter                                   Boric acid    H.sub.3 BO.sub.3                                                                             10     g/liter                                   Ethylene glycol                                                                             HO--CH.sub.2 --CH.sub.2 --OH                                                                 20     g/liter                                   Sodium sulfate                                                                              Na.sub.2 SO.sub.4                                                                            25     g/liter                                   Polyvinyl pyridinium-N-                                                       sulfopropyl betaine          0.8    g/liter                                   pH value 5.9                                                                  Temperature 20°C                                                       ______________________________________                                    

The high-luster coatings have a hardness of HV₀.01 = 140-150 kp/mm².

EXAMPLE 5

    ______________________________________                                        Silver as    AgSCN or AgCN  20      g/liter                                   Sodium thiosulfate                                                                         Na.sub.2 S.sub.2 O.sub.3.5H.sub.2 O                                                          165     g/liter                                   Sodium tetraborate                                                                         Na.sub.2 B.sub.4 O.sub.7.10H.sub.2 O                                                         18      g/liter                                   Dipropylene triamine                                                                       H.sub.2 N--(CH.sub.2).sub.3 --NH--                                            (CH.sub.2).sub.3 --NH.sub.2                                                                  5       g/liter                                   Polyethylenimine                                                                           mol.wt. >1000  0.2     g/liter                                   pH value 8.0                                                                  Temperature 20-30°C                                                    ______________________________________                                    

There result bright, ductile silver coatings. The presence of SCN ionshas a favorable effect on the anodic dissolution of silver.

EXAMPLE 6

    ______________________________________                                        Silver as      Ag.sub.2 O     34     g/liter                                  Sodium thiosulfate                                                                           Na.sub.2 S.sub.2 O.sub.3.5H.sub.2 O                                                          375    g/liter                                  Sodium bicarbonate                                                                           NaHCO.sub.3    10     g/liter                                  Diselenium-diglycolic acid                                                                   (HOOC--CH.sub.2 --Se--).sub.2                                                                0.4    g/liter                                  pH value 9.9                                                                  Temperature 20-23°C                                                    ______________________________________                                    

The bath furnishes excellently bright, ductile coatings, especially inhigher current density ranges (over 1.5 A/dm2).

EXAMPLE 7 ##STR5##

Silver coatings deposited from this bath have high luster, are ductileand have a specific electric conductivity of about 40 m Ohm mm2.

EXAMPLE 8

    ______________________________________                                        Silver as       Na.sub.3 Ag(S.sub.2 O.sub.3).sub.2                                                        22      g/liter                                   Sodium thiosulfate                                                                            Na.sub.2 S.sub.2 O.sub.3.5H.sub.2 O                                                       110     g/liter                                   Secondary sodium phosphate  50      g/liter                                   Primary sodium phosphate    10      g/liter                                                   Mol.wt. from                                                  Polyethylenimine                                                                              500 to 1000                                                   reacted with                                                                  Epichlorhydrin                                                                and quaternized with                                                          Dimethyl sulfate            0.75    g/liter                                   pH value 7.4                                                                  Temperature 16-25°C                                                    ______________________________________                                    

The coatings deposited from this bath are remarkable especially for highhardness (over 170 kp/mm²).

EXAMPLE 9

    ______________________________________                                        Silver as    Na.sub.3 Ag(S.sub.2 O.sub.3).sub.2                                                           26      g/liter                                   Ammonium thiosulfate                                                                       (NH.sub.4).sub.2 S.sub.2 O.sub.3                                                             250     g/liter                                   Boric acid   H.sub.3 BO.sub.3                                                                             30      g/liter                                   Ethylene glycol                                                                            HO--CH.sub.2 --CH.sub.2 --OH                                                                 60      g/liter                                   Reaction product of                                                           dipropylene triamine                                                          with epichlorhydrin         0.3     g/liter                                   pH value 6.3                                                                  ______________________________________                                    

Reaction products of tetraethylene pentamine, dimethylamino-propylamineand N,N'-bis(4-hydroxybutyrul)-tripropylene triamine with epihalohydrinsgive analogous results, namely high-luster coatings with a specificelectric resistance of about 3.6 μ Ohm cm.

EXAMPLE 10

    ______________________________________                                        Silver as        Ag.sub.2 CO.sub.3                                                                        30      g/liter                                   Sodium thiosulfate                                                                             Na.sub.2 S.sub.2 O.sub.3 5H.sub.2 O                                                      300     g/liter                                   Sodium sulfite   Na.sub.2 SO.sub.3                                                                        40      g/liter                                   Sodium bisulfite NaHSO.sub.3                                                                              5       g/liter                                   Reaction product of                                                           1,3-dichloropropane-2-ol with                                                 1,8-diaminooctane           0.9     g/liter                                   pH value 10.2                                                                 Temperature 20-30°C.                                                   ______________________________________                                    

This electrolyte furnishes bright, ductile coatings with a hardness ofHV₀.01 = 150-160 kp/mm2 and a specific electric resistance of 2.1 μ Ohmcm.

I claim:
 1. In a cyanide-free aqueous bath for electrodeposition ofsilver containing silver and a thiosulfate, the improvement whichcomprises the presence in said bath of at least one additive selectedfrom the group consisting ofa. an organic nitrogen compound having atleast two nitrogen atoms and a molecular weight of at least 300 selectedfrom the group consisting of polyethyleneimine, polypropyleneimine,dipropylene triamine, polyvinyl pyridinium-N-sulfopropyl-betaine,polyethyleneimine reacted with epichlorhydrin and quaternized withdimethyl sulfate, a reaction product of 1,3-dichloropropane-2-ol with1,8-diamino-octane, a reaction product of a member of the groupconsisting of dipropylene-triamine, tetra-ethylenepentamine,dimethylamino-propylamine andN,N'-bis(4-hydroxybutyryl)-tripropylene-triamine with epichlorhydrin,and a reaction product of a member of the group consisting of1,4-dichlorobutane, 1,4-dibromobutane and 1,3-dibromopropane with amember of the group consisting of tetramethylethylenediamine,tetraethylenediamine and tetramethylpropylene-diamine; and b. a compoundselected from the group consisting of diselenium-diglycolic acid anddiphenyldisulfide-2,2'-dicarboxylic acid,said additive and compound eachbeing present in an amount from about 10⁻ ⁸ to 0.5 mole per liter ofbath.
 2. The improvement of claim 1 wherein the nitrogen compound is thereaction product of 1,4-dichlorobutane with tetramethylethylene-diamine.3. The improvement of claim 1 wherein the nitrogen compound ispolyethyleneimine and the selenium compound is diselenium diglycolicacid.